<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .slider-bar {
            position: absolute;
            left: 50%;
            top: 300px;
            margin-left: 600px;
            width: 45px;
            height: 130px;
            background-color: pink;
        }

        .w {
            width: 1200px;
            margin: 10px auto;
        }

        .header {
            height: 150px;
            background-color: purple;
        }

        .banner {
            height: 250px;
            background-color: skyblue;
        }

        .main {
            height: 1000px;
            background-color: yellowgreen;
        }

        span {
            display: none;
            position: absolute;
            bottom: 0;
        }
    </style>
</head>

<body>
<div class="slider-bar">
    <span class="goBack">返回顶部</span>
</div>
<div class="header w">头部区域</div>
<div class="banner w">banner区域</div>
<div class="main w">主体部分</div>
<script>
    //1.获取元素
    var sliderbar = document.querySelector('.slider-bar');
    var banner = document.querySelector('.banner');
    var main = document.querySelector('.main');
    var goBack = document.querySelector('.goBack');
    //banner.offsetTop被卷去头部的大小 一定要写在滚动的外面
    var bannerTop = banner.offsetTop;
    //当我们侧边栏固定定位之后应该变化的数值
    var sliderbarTop = sliderbar.offsetTop - bannerTop;
    var mainTop = main.offsetTop;
    console.log(bannerTop);
    //2.页面滚动事件 scroll
    document.addEventListener('scroll',function () {
        //window.pageYOffset 页面被卷去的头部
        // console.log(window.pageYOffset);
        //3.当我们页面被卷去的头部大于等于了172此时,侧边栏就要改为固定定位
        if (window.pageYOffset >= bannerTop){
            sliderbar.style.position = 'fixed';
            sliderbar.style.top = sliderbarTop + 'px';
        }else{
            sliderbar.style.position = 'absolute';
            sliderbar.style.top =  '300px';
        }
        //4.当页面滚到到main盒子,就显示goBacK模块
        if (window.pageYOffset >= mainTop){
            goBack.style.display = 'block';
        }else {
            goBack.style.display = 'none';
        }
    })
</script>
</body>
</html>
